Such a crystallizer is known from the Dutch patent specification No. 146,394. This known crystallizer is in the shape of a trough and between the discs attached to the shaft are arranged cooling bodies which reach downwardly to the shaft. The discs leave a large space free between the discs and the wall of the trough. The cooling bodies are placed at a small angle and therefore a conveying action is obtained so that the slurry with crystals flows from the one section to the next downstream section. This known device can be used in a sense that it works effectively only with highly viscous solutions. Only with high viscosity solutions one has no problems with the formation of crystals on the cooled surfaces. Moreover, due to the high viscosity there is no problem of a local back flow of the solution to the next preceding or upstream section. The tendency to formation of crystals on the surfaces is defined by the following factors. The amount of heat withdrawn per unit of the cooling surface, the width of the metastable region of the relevant solution, the viscosity of the solution and of the produced slurry of crystals and the roughness of the cooled surfaces. In the case of a high viscosity, one has as a rule a broader metastable region, whereas the crystals will also settle less quickly, so that it is easier to have a homogeneous distribution of the crystals in the slurry.
It is also possible to have a larger cooling capacity per unit of cooling surface. On the other hand, with a low viscosity solution, one has as a rule a narrow metastable region and the crystals will settle quickly. With such a solution there is a considerably lower cooling capacity per unit of cooling surface possible and per ton of produced crystal it is necessary to install a considerably larger and more costly cooling surface. This is the reason that the known crystallizer can practically only be used in such cases, where one has crystallization in highly viscous solutions and where a relatively small amount of heat has to be withdrawn. This is for example the case when one has to aftercool sugar-syrup in a sugar factory.
The backflow in the known device when used with low viscosity liquids has for its result that a departure of the cooling profile occurs so that a product of inferior quality is obtained and that the formation of crystals at the cooling surfaces is promoted. This again has as its result that there is a larger resistance to the flow of heat and a smaller heat flux. In the case of a certain heat flux with a low viscosity solution in the vicinity of the cooled wall surface and in the case of a slowly moving solution the temperature profile can be such that the metastable region is passed so that an undesirable amount of crystal grains occurs and an increased formation of crystals on the wall surface will occur. From the above it will be clear that the known crystallizer cannot be used in the case of low viscosity solutions.
The invention has for its aim to provide a crystallizer as described above which is suited for solutions of low viscosity.
According to the invention the discs over the larger part of its circumference fit with small clearance to the wall of the vessel or to a portion connected with that wall and on the shaft in each section scrapers have been provided which touch along the cooled wall surfaces.
With such a device the cooled wall surfaces are kept free from crystals adhering to these surfaces and because the scrapers also have the function of stirrers, one has in each section a good mixing so that the crystals are distributed homogeneously and in the case of a certain heat flux a less steep temperature profile in the vicinity of the cooled wall surface is obtained, so that it is prevented that crystals are formed as a result of passing the metastable region. Owing to the stirring effect of the scrapers one has moreover a good withdrawal of heat from the solution in each section. The crystallizer according to the invention can also be used for crystallization from a clear liquid in which no crystal seed is present. In the first section the solution is then strongly undercooled so that grains are formed and in the next sections such an amount of heat is withdrawn that the solution stays in the metastable region and no new crystals are formed but only the already existing crystals are growing.
According to the invention the vessel can have the shape of a trough and in the trough there are arranged filling pieces having outer edges which fit to the wall of the trough, an upper edge extending above the liquid surface and an inner edge fit with small clearance to the outer circumference of the discs arranged on the shaft. In such an embodiment the section of the trough does not necessarily have to be round and due to the filling pieces one has a good separation between the sections enclosed between the discs.
In an advantageous embodiment according to the invention in each section between the discs a disc-like cooling element is provided, the cooling surfaces of this element being directed perpendicular to the shaft and on the shaft scrapers have been attached which touch the cooling surfaces of the cooling elements with scaper edges directed perpendicular to the shaft. Further according to the invention the cooling elements have a recess directed in the radial and vertical direction, said recess receiving the shaft. In such an embodiment it is easy to remove the cooling elements and to clean these elements in case of operational faults or crystals adhering to the cooling elements.
According to the invention the vessel can have a double wall and the chamber enclosed by the double walls is used as a cooling chamber, said chamber by transverse partitions being divided in sections succeeding to each other in axial direction and the discs are provided with scraper edges extending in axial directions, said scraper edges touching the cooled wall of the vessel. The sections between the discs mounted on the shaft can now be completely free so that only the scrapers are moving in the sections and also are acting as stirrers.
According to the invention, the scraper edges of the succeeding discs are displaced in circumferential direction in relation to each other. This has for its result that in each section a stirring action is obtained in different places seen in circumferential direction.
According to the invention, the length of the axially directed scraper edges is such that the scraper edges of the succeeding discs overlap in axial direction. Here one is such that the whole wall surface is scraped in an efficient way.
According to the invention, the passage openings in the discs consist of sector-shaped or segment-shaped recesses. This has for its result that on only one place of the disc is a passage near the circumference of the vessel, and this passage opening is displaced continuously along the circumference of the vessel.
According to the invention the succeeding discs provided with passage openings are arranged in such a way that the passage openings in circumferential direction are rotated in relation to each other. The risk that a section is short circuited is reduced.
In an efficient embodiment according to the invention the succeeding discs are rotated over 180.degree. in relation to each other. Here it is practically impossible that the liquid from a preceding section is flowing through a section directly to the next section.
The liquid has to cover a relatively long path inside a section and during the flowing along this path, the liquid is sufficiently stirred.
According to the invention, the double walled vessel can be cylindrical and closed. This makes it possible to handle solutions in the crystallizer which must be held out of contact with the atmosphere.